I sell a zipped download product on my website, and two times now people have said it was incompatible with their system. I don't have any more details than that. Does anyone have experience with this? I thought the zip format was universal, but perhaps not?

I find it pretty reliable. What system are you doing this on? A Mac? As a Mac user myself, I discovered that when I zip files, the Mac includes Mac-only files that confuse people on PCs. So now I use a little free program called YemuZip to do my zipping. (Of course, if you are not on a Mac yourself, this info is totally irrelevant to your question. )